Defibrillators – A Measure For Life Saving
The most common form of treatment for any form of life threatening cardiac
arrhythmias and pulsless tachycardia is defibrillation, this is the process of
giving a measured amount of electrical energy to the patients heart by way of a
defibrillator.
It is this electrical current that will halt the arrhythmia and thereby enabling
the heart to return to a normal rhythm. There are three forms of systems, either
implanted, transvenous or external. With most external systems the current is
automated , this enables the untrained bystander or layman to utilize them
successfully.
Cardiopulmonary resuscitation or the more common term CPR is
seen as a temporary treatment that will maintain a minimum flow of blood to the
brain. By using a defibrillator the patient will receive a controlled electrical
shock that will take the heart from a threatening rhythm such as VF or
ventricular fibrillation and restore the normal paced rhythms. The system will
retain the voltage and pass it to the patient by way of 2 paddles, or electrodes
that are placed on the chest, depending on the system.
Up until the early 1950’s the only way that the heart was shocked , was when
the patients chest was open and the electrodes were placed on either side of the
exposed heart, this method used an alternating current (AC) of 300 or greater
volts. AC current in excess of 1000 volts was applied by using electrode to the
closed chest of the patient – this method was introduce late 1950’s.
It was Bernard Lown who in 1959 began researching another technique that
charged capacitors up to almost 1000 volts, this having an energy level of
between 100-200 joules, thereafter a very heavy exactly measured sinusoidal wave
of 5 milliseconds is delivered to the heart by paddle electrode, this is the
monophasic wave form.
To understand how monophasic waveforms work we need to know that the current
will flow in one direction only from one paddle to the other, this will in fact
cause the heart to stop and to restart under its own cognizance. This monophasic
method was used for many years, and it thereafter that a biphasic waveform was
developed, unlike the monophasic, here the current will flow from one paddle to
the other then n the reverse for the second or biphasic.
With modern day research showing that biphasic waveforms have proved to be
more effective than the elder monophasic method also when the energy levels are
the same it is for this reason that the manufacturers of the external
defibrillators are using the biphasic waveform as a standard.
It is clear that for a successful defibrillation to take place that a
biphasic waveform lowers the threshold, however the underlying mechanisms are
still not fully understood.
Currently the energy levels of defibrillators vary from one to another
manufacturer depending on the model. With some systems the energy levels are
constant whereas with other devices they are programmable and the exact level
can be selected by the user depending on the circumstances at hand. With none or
very little comparative data, the manufacturers are unable to make
recommendation as to the most effective energy level or biphasic waveform.
Defibrillators – Modern Technology
Modern technology , with the assistance of previous studies based on the
implanted device have led to the advent of the Automated External Device (AED)
this system has the capacity to analyze the rhythm of the heart and then it will
calculate the required charge, thus limiting the clinical skill required and
letting emergencies get treated effectively and speedily.
Based on simple computer technology, the above (AED) system has been designed
to analyze the heart and then let the user know if a shock is in fact required
and if so how much. Being specially designed for the layman with little or no
training at all and are somewhat limited in their use for medical practitioners’
who when treating will be required to make a diagnosis and will therefore use a
manual or semi automated system.
A problem that is associated with the automatic system is that the ceasing of
CPR in order to shock the patient has shown to have a negative effect on the
defibrillation process it is here that a professional that is on hand could make
the diagnosis and regulate the shock accordingly.
Currently there are two different types of Automated External Devices (AED),
the semi and the fully automatic models. The fully automated system will after
diagnosing the problem will administer the shock wave automatically, whereas the
semi- automatic will after diagnosis will determine is a shock is indeed
required and if so the user will need to manually press a button to administer
the measure shock. There are additional features depending on the model, where
an ECG monitor can be purchased or a manual override.
The electronic circuit of the biphasic defibrillator can be
designed around a high voltage-high capacity energy storing condenser. However,
contrary to the common monophasic defibrillators it will have two different
discharge routes: a system of four high voltage-high current IGBT’s is switched
alternatively in two opposite directions – to provide the biphasic discharge
wave. (The IGBT’s configuration resembles a four diodes conventional rectifier
bridge).
Source: http://www.medwow.com/articles/
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